1. Field of the Invention
This invention relates to generalised connection networks of particular, but not exclusive, application to optical networks.
2. Related Art
Optical space switches can provide broadband switched connections in PBX and local network environments. Generalised connection networks (GCNs) offer the additional facility of broadcasting, for example, from any one of N inputs to any number up to N of outputs enabling any customer in a local network, for example to become a broadcast service provider to any combination of the other customers. The smallest GCNs published to date operate by separating the broadcast function into two parts; an initial replication network (generaliser) to generate the required number of copies, followed by a one-to-one switching network (connector) to connect the copies to the appropriate outputs. This segregated approach requires more crosspoint switches than the Nlog.sub.2 N theoretically needed to provide all of the N.sup.N possible permutations. GCNs using make/break contacts require 5.8Nlog.sub.2 N, and when using optical changeover switches they require in the order of 2Nlog.sub.2 N-N+2.
FIG. 2 of Sakata et al "Synthesis of Multiconnected Switching Networks, Electronics and Communications in Japan, vol, 58-A, no. 1, 51-58 (1975)", shows a GCN in which a branching network is positioned between a left-hand and a right-hand sub-network made up from two input, two output, 2-state branching elements whose task is either to allow both inputs through unaltered, or to copy one input, the upper input, to both outputs. In optics, where 2.times.2 changeover switches naturally perform a crossover function, their use as branching elements requires disconnection of the lower input line when copying, and this is easily provided by turning off the light source not seeking connection through the network or by adding a net of N additional on/off switches. A signal at a given input can be copied to a subset of outputs of the branching network as desired by passing sequentially through the switches in a downward direction. The right hand permutation network then interconnects the output of the branching network to the desired outputs of the GCN.